Methods of making quantum dot films

ABSTRACT

In an example embodiment, an optical device includes an integrated circuit, an array of conductive regions, and an optically sensitive material over at least a portion of the integrated circuit and in electrical communication with at least one conductive region. In another example embodiment, a method of forming a nanocrystalline film includes fabricating nanocrystals having a plurality of first ligands attached to their outer surfaces, exchanging the first ligands for second ligands of a different chemical composition, forming a film of the ligand-exchanged nanocrystals, removing the second ligands, and fusing the cores of adjacent nanocrystals in the film to form an electrical network of fused nanocrystals. In another example embodiment, a film includes a network of fused nanocrystals with at least portions of the fused nanocrystals being in direct physical contact with adjacent nanocrystals, the film having substantially no defect states in regions where cores of the nanocrystals are fused.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/780,026, filed May 14, 2010, which is a divisional patent applicationof U.S. patent application Ser. No. 11/509,318, filed Aug. 24, 2006, nowissued as U.S. Pat. No. 7,746,681, which claims the benefit of priorityunder 35 U.S.C. §119(e) of U.S. Provisional Application Ser. No.60/710,944, filed Aug. 25, 2005, and which is also acontinuation-in-part of U.S. application Ser. No. 11/327,655, filed Jan.9, 2006, which claims priority from U.S. Provisional Application Ser.No. 60/641,766, filed Jan. 7, 2005, all of which are incorporated hereinby reference in their entireties.

This application is also related to the following applications:

U.S. patent application Ser. No. 11/510,510, filed on Aug. 24, 2006, andentitled “Quantum Dot Optical Devices with Enhanced Gain and Sensitivityand Methods of Making Same,” now issued as U.S. Pat. No. 7,773,404;

U.S. patent application Ser. No. 11/510,263, filed on Aug. 24, 2006, andentitled “Electronic and Optoelectronic Devices with Quantum Dot Films,”now issued as U.S. Pat. No. 7,742,322;

U.S. patent application Ser. No. 11/108,900, filed Apr. 19, 2005 andentitled “Optically Regulated Optical Emission Using Colloidal QuantumDot Nanocrystals,” now issued as U.S. Pat. No. 7,326,908; and

U.S. Provisional Application Ser. No. 60/563,012, filed Apr. 19, 2004and entitled “Multi-Color Optical and Infrared Emission Using ColloidalQuantum Nanocrystals.”

BACKGROUND

1. Field of the Invention

The present invention generally relates to optical and electronicdevices including nanocrystals, such as quantum dots.

2. Description of Related Art

Many systems currently used for short-wavelength infrared (SWIR)photodetection and imaging are achieved through epitaxial growth ofcompound semiconductors such as InGaAs, or chemical bath growth ofpolycrystalline PbS or PbSe. These techniques can result inexceptionally sensitive detectors—normalized detectivity, D*, as high as8×10¹⁰ Jones from PbS at room temperature for example—but theirdeposition is generally incompatible with established silicon integratedcircuit fabrication techniques. In such systems a silicon electronicread-out array and an infrared-sensitive photo detector array arefabricated separately. This non-monolithic process then necessitates acomplex assembly procedure, resulting in low yield, poor resolution(e.g., at least 10× lower pixel count than a low-cost commercial siliconcamera), and high cost (e.g., at least 100× greater than a siliconcamera).

SWIR photodetection and imaging may also be achieved using quantum dotsas a photosensitive material; however, imaging systems using quantumdots typically have relatively low gains and sensitivities. Someexamples of imaging systems that utilize quantum dots, and applicationsthereof, may be found in the incorporated references given below.

A schematic of a ligand-capped QD nanocrystal is illustrated in FIG. 1.The QD includes a core 100, which includes a highly crystallinesemiconductor region of relatively small size, e.g., from about 1-10 nm,for example about 5 nm as shown in the figure. The core is typicallyhighly or may even be perfectly crystalline, is known to have asubstantially homogeneous structure and composition. The QD issurrounded by a plurality of ligands 120 attached to its outer surface.Specifically, each ligand 120 includes a long chain, represented by thejagged line, and an end functional group 150, represented by thetriangle, which connects the ligand to the outer surface of the QD.

The fabrication in solution of QDs, stabilized using suitable ligands,and typical QD characteristics such as size-tunable absorbance andemission are known. Solution-fabricated QDs may be referred to as“colloidal,” as compared with epitaxially-grown (e.g.,Stranski-Krastanov-mode grown) or otherwise deposited QDs. Furtherdetails may be found in the incorporated references included below.

SUMMARY

The inventions, embodiments of which are described here, have a numberof aspects including an imaging system, a focal plane array whichincludes an optically sensitive layer formed on an underlying circuit(e.g., a read-out structure which includes an integrated circuit)patterned to measure and relay optical signals, electronic signals, orboth, on a pixel-by-pixel basis, where the signal is indicative of lightabsorbed in the medium from which the focal plane array is made. Thecircuit achieves multiplexing of the values read from individual pixelsinto row or columns of data, carried by electrodes. Subsequent layers,typically processed from the solution phase, which, with appropriateinterfacing, sensitize the underlying focal plane array to becomeresponsive to the wavelengths absorbed by these new layers. Theirresultant electronic signals are registered and relayed using theunderlying chip.

A range of structures can be formed on an integrated circuit of theread-out structure that enable the medium from which the chip itself ismade, and also the optically sensitive layer, to be electronicallybiased and their resultant signals read by the circuit.

The invention provides a range of solution-processed optically sensitivelayers that would lie atop the underlying chip. In a particularembodiment, the invention provides a method of sensitizing a silicon CCD(charge-coupled device) or CMOS focal plane array into the infraredspectral range using thin films which include spin-coated quantum dotnanocrystals. The invention includes a method of sensitizing apre-fabricated focal plane array sensitive into the visible and infraredspectral ranges using spin-coated quantum dot nanocrystals andsemiconducting polymers.

Thus, efficient, high-detectivity photodetectors based onsolution-processed quantum dots with subsequent solution-phase andvapor-phase thermal processing have been produced. Also manufacturaableare highly sensitive photodetectors based on a combination of two (ormore) types of solution-processed quantum dots, each composed of adistinct semiconductor material. In addition, efficient,high-detectivity photo detectors based on a combination ofdifferently-treated solution-processed quantum dots may be constructed.

In some embodiments, the imaging devices are efficient photoconductiveoptical detectors active in the x-ray, ultraviolet, visible,short-wavelength infrared, long-wavelength infrared regions of thespectrum, and are based on solution-processed nanocrystalline quantumdots. Certain of these embodiments have the potential to be used increating low-cost infrared imaging systems for security, night vision,and missile tracking applications, while other embodiments have thepotential to be used in other kinds of imaging systems.

In other aspects, the inventions include methods and structures forforming useful QD structures, typically in the form of a film. Themethods include fabricating a plurality of nanocrystals, each having acore and an outer surface with a plurality of first ligands having afirst length being attached to the outer surface. The ligands attachedto the outer surface of the nanocrystals are replaced with a pluralityof second ligands having a second length less than the first length. Afilm of ligand-exchanged nanocrystals is formed, such that at least aportion of the ligand-exchanged nanocrystals are adjacent at least oneother ligand-exchanged nanocrystal. The second ligands attached to theouter surfaces of the nanocrystals of the film of ligand-exchangednanocrystals are removed—either partially, substantially, orcompletely—so as to bring the outer surfaces of adjacent nanocrystalsinto closer proximity, and even to cause “necking” or touching betweenthe nanocrystals. The cores of adjacent nanocrystals can be furtherfused to form an electrical network of fused nanocrystals. The film canhave defect states on the outer surfaces where the cores are not fused,formed, for example, through oxidation. The film thus produced can beused as part of a sensor, or formed over a device used as part of asensor.

In other aspects, the inventions include devices with improvedproperties. In one embodiment, a device is provided with a noiseequivalent exposure (NEE) of less than 10⁻¹¹ J/cm² at wavelengths of 400nm to 800 nm, and further less than 10⁻¹⁰ J/cm² at wavelengths of 400 nmto 1400 nm. In other embodiments, a device has a responsivity asmeasured in A/W of between about 1 and about 1,000, or even betweenabout 1 and about 10,000, for example at least 100, or preferably morethan 1000, or still more preferably at greater than 10,000. Theresponsivity is a function in part of the bias voltage applied, with agreater responsivity with higher bias. In still other embodiments, adevice provides a substantially linear response over 0-10V with a biasapplied across a distance of 0.2 to 2 microns width or gap. A device canbe produced with a combination of these properties.

Under one aspect, a device includes an integrated circuit having anarray of conductive regions; and an optically sensitive material over atleast a portion of the integrated circuit and in electricalcommunication with at least one conductive region of the array ofconductive regions.

One or more embodiments include one or more of the following features.The optically sensitive layer includes an array of islands of opticallysensitive material, wherein a plurality of the islands overlay acorresponding plurality of the conductive regions. The integratedcircuit includes three-dimensional features and wherein the opticallysensitive material conforms to at least a portion of saidthree-dimensional features. Further including an electrode overlayingand in electrical communication with at least a portion of the opticallysensitive layer. The electrode is at least partially transparent. Theelectrode includes at least one of a band-pass and a band-blockmaterial. The conductive regions are arranged in one or more rows overthe integrated circuit. The conductive regions are further arranged inone or more columns over the integrated circuit. The conductive regionsare arranged in a plurality of rows and columns over the integratedcircuit. The integrated circuit includes a flexible substrate and isformed in a non-planar shape. The integrated circuit includes at leastone of a semiconducting organic molecule and a semiconducting polymer.The optically sensitive layer includes a plurality of nanocrystals. Theoptically sensitive layer includes a plurality of fused nanocrystals,each nanocrystal having a core and an outer surface. The outer surfacesof the fused nanocrystals are at least partially free of ligands. Theoptically sensitive layer includes a continuous film having nanoscalefeatures, the nanoscale features comprising an interconnected network offused nanocrystals, wherein substantially each fused nanocrystalincludes a core in direct physical contact and electrical communicationwith the core of at least one adjacent nanocrystal. The continuous filmis substantially inorganic. The continuous film includes ligands onportions of the outer surface excluding portions where the nanocrystalshave been fused. The outer surface of substantially each fusednanocrystal includes a material having a different composition from thecore. The outer surface of substantially each fused nanocrystal includesoxidized core material. The outer surface of substantially each fusednanocrystal includes semiconductor material. The outer surface ofsubstantially each fused nanocrystal includes at least one defect state.The optically sensitive layer includes an optically active polymer. Theoptically active polymer includes at least one of MEH-PPV, P3OT, andP3HT. The conductive regions include pixel regions, and wherein theintegrated circuit includes a readout circuit capable of activating apixel region by applying an electrical signal to a control lead incommunication with that pixel region so that current flows through theoptically sensitive layer and the pixel region, wherein the amount ofcurrent that flows through the optically sensitive layer and the pixelregion is related to a number of photons received by the opticallysensitive layer. The integrated circuit includes a CMOS active pixel.The integrated circuit includes a CCD pixel. During operation an amountof current flowing in the optically sensitive layer is substantiallylinearly related to an amount of light received by the opticallysensitive layer over at least a portion of its intended operating range.The optically sensitive layer has a photoconductive gain of betweenabout 1 and 1,000 A/W, or between about 1 and 10,000 A/W, or at leastabout 10,000 A/W, or between about 100 and 10,000 A/W. The opticallysensitive layer has a noise equivalent exposure of less than about 10⁻¹¹J/cm² between the wavelengths of 400 nm and 800 nm, or between about10⁻¹¹ and 10⁻¹² J/cm² between the wavelengths of 400 nm and 800 nm, orless than about 10⁻¹⁰ J/cm² between the wavelengths of 400 nm and 1400nm, or less than about 10⁻¹¹ J/cm² in at least a portion of the spectrumbetween the wavelengths of 10 nm and 5 μm, or less than about 10⁻¹²J/cm² in at least a portion of the spectrum between the wavelengths of10 nm and 5 μm. The optically sensitive layer has an electricalresistance of greater than about 25 k-Ohm/square. The opticallysensitive layer has a carrier mobility of between about 0.001 and about10 cm²/Vs, or between about 0.01 and about 0.1 cm²/Vs, or greater thanabout 0.01 cm²/Vs.

Under another aspect, a method of making a device includes providing anintegrated circuit having a top surface and an array of electrodeslocated therein, at least some of the electrodes being arranged toconvey signals from the array to an output; and solution-depositing anelectrically active layer onto at least a portion of the top surface ofthe integrated circuit such that it is in direct and continuouselectrical contact with said at least a portion.

One or more embodiments include one or more of the following features.Solution-depositing the electrically active layer includesspray-coating, dip-casting, drop-casting, evaporating, blade-casting, orspin-coating the electrically active layer onto the top surface of theintegrated circuit. Patterning the electrically active layer. Patterningincludes lithographically patterning after it is solution-deposited.Patterning includes self-assembling the electrically active layer ontoone or more selected regions of said at least a portion. Patterningincludes depositing the electrically active layer over protrusions andtrenches in the integrated circuit and then planarizing the electricallyactive layer to remove portions of the layer from the protrusions andleaving portions of the layer in the trenches. The array of electrodesincludes three-dimensional features and the electrically active layerconforms to the three-dimensional features. Solution-depositing theelectrically active layer includes solution-depositing nanocrystals,each nanocrystal having a core and an outer surface. The nanocrystalshave a size between about 1-10 nm. The nanocrystals include nanocrystalsof different compositions. The nanocrystals include nanocrystals ofdifferent sizes. The nanocrystals are substantially monodisperse. Thenanocrystals include at least one of PbS, InAs, InP, PbSe, CdS, CdSe,In_(x)Gal_(1-x)As, (Cd—Hg)Te, ZnSe(PbS), ZnS(CdSe), ZnSe(CdS), PbO(PbS),and PbSO₄(PbS). Also fusing at least a portion of the nanocrystals toeach other after solution-depositing them. Fusing at least a portion ofthe nanocrystals to each other includes removing ligands from the outersurface of said at least a portion of the nanocrystals. Fusing at leasta portion of the nanocrystals to each other includes removing at least aportion of the ligands from the outer surface of said at least a portionof the nanocrystals; and annealing the nanocrystals so as to fuse thecores of said at least a portion of the nanocrystals to other cores ofsaid at least a portion of the nanocrystals Annealing the nanocrystalsremoves at least a portion of the ligands from the outer surface of saidat least a portion of the nanocrystals. Annealing the nanocrystalsincludes heating them to a temperature between about 150° C. and about450° C. Annealing the nanocrystals includes heating them to atemperature between about room temperature and about 150° C. Alsoperforming a ligand-exchange on at least a portion of the nanocrystalsbefore solution-depositing them so as to provide relatively shortligands on said at least a portion of the nanocrystals. The relativelyshort ligands include at least one of pyridine, allylamine, methylamine,ethylamine, propylamine, butylamine, octylamine, and pyrrolidineligands. The electrically active layer is also optically sensitive. Atleast some of the electrodes of the integrated circuit are configured todefine optical pixels which are read by others of the electrodes. Alsoselecting a wavelength region of the electromagnetic spectrum in whichthe electrically active layer is intended to operate. Selecting thewavelength region includes selecting nanocrystals of a particular sizeand including them in the electrically active layer. The wavelengthregion includes at least one of the x-ray, infrared, visible, andultraviolet regions of the electromagnetic spectrum. The electricallyactive layer includes a semiconducting polymer. The semiconductingpolymer includes at least one of MEH-PPV, P3OT, and P3HT. Also providingat least one electrode over and in electrical contact with at least aportion of the electrically active layer. The at least one electrode isat least partially optically transparent. The at least one electrodeincludes at least one of a bandpass filter and a bandblock filter. Theat least one electrode includes at least one of indium tin oxide, indiumoxide, tungsten oxide, aluminum, gold, platinum, silver, magnesium,copper, and combinations and layer structures thereof. Also providing ananti-reflection coating over the electrically active layer. Alsoproviding a protective coating over the electrically active layer forprotecting the layer from one or more environmental influences. Alsoproviding an optical filter coating over the electrically active layer,wherein the optical filter includes at least one of a bandpass filterand a bandstop filter. The integrated circuit includes a flexiblesubstrate and is formed in a non-planar shape. The integrated circuitincludes at least one of a semiconducting organic molecule and asemiconducting polymer. The integrated circuit includes at least one ofsilicon, silicon-on-insulator, silicon-germanium, indium phosphide,indium gallium arsenide, gallium arsenide, glass, and polymer.

Under another aspect, a device includes a plurality of electrodes; andan optically sensitive layer between, in contact with, and in electricalcommunication with the electrodes, the electrodes for providing a signalindicative of radiation absorbed by the optically sensitive layer, theoptically sensitive layer providing a photoconductive gain of at leastabout 100 A/W.

One or more embodiments include one or more of the following features.The optically sensitive layer has a photoconductive gain of at leastabout 1000 A/W. The optically sensitive layer has a photoconductive gainof at least about 10,000 A/W. The optically sensitive layer has aphotoconductive gain of between about 100 and 10,000 A/W.

Under another aspect, a device includes a plurality of electrodes; andan optically sensitive layer between, in contact with, and in electricalcommunication with the electrodes, the electrodes for providing a signalindicative of radiation absorbed by the optically sensitive layer,wherein the optically sensitive layer has a noise equivalent exposure ofless than about 10⁻¹¹ J/cm² at wavelengths between 400 nm and 800 nm.

One or more embodiments include one or more of the following features.The optically sensitive layer has a noise equivalent exposure of betweenabout 10⁻¹¹ and 10⁻¹² J/cm² at wavelengths between 400 nm and 800 nm.The optically sensitive layer has a noise equivalent exposure of lessthan about 10⁻¹⁰ J/cm2 at wavelengths between 400 and 1400 nm. Theoptically sensitive layer has a photoconductive gain of at least about100 A/W. The optically sensitive layer has a photoconductive gain of atleast about 1000 A/W. The optically sensitive layer has aphotoconductive gain of at least about 10,000 A/W.

Under another aspect, a device includes a plurality of electrodes; andan optically sensitive layer between, in contact with, and in electricalcommunication with the electrodes, the electrodes for providing a signalindicative of radiation absorbed by the optically sensitive layer,wherein the optically sensitive layer has a carrier mobility of greaterthan about 0.001 cm²/Vs.

One or more embodiments include one or more of the following features.The ptically sensitive layer has a carrier mobility of between about0.01 cm²/Vs and about 0.1 cm²/Vs. The optically sensitive layer has acarrier mobility of up to about 10 cm²/Vs.

Under another aspect, a method of forming a nanocrystalline filmincludes fabricating a plurality of nanocrystals, the nanocrystalshaving a core and an outer surface, a plurality of first ligands havinga first length being attached to the outer surface; exchanging theplurality of first ligands attached to the outer surface of thenanocrystals for a plurality of second ligands having a second lengthand having a different chemical composition than the plurality of firstligands; forming a film of ligand-exchanged nanocrystals, wherein atleast a portion of the ligand-exchanged nanocrystals are adjacent atleast one other ligand-exchanged nanocrystal; removing the secondligands attached to the outer surfaces of the nanocrystals of the filmof ligand-exchanged nanocrystals so as to bring the outer surfaces ofadjacent nanocrystals into closer proximity; and fusing the cores ofadjacent nanocrystals so as to form an electrical network of fusednanocrystals.

One or more embodiments include one or more of the following features.Fabricating a plurality of nanocrystals includes forming thenanocrystals in a substantially inert environment so as to substantiallyprevent the formation of defect states on the outer surfaces of thenanocrystals. The second length is less than the first length. The firstligands each include a carbon chain greater than about 10 carbons long.The second ligands each include a carbon chain between about 1-10carbons long. The second ligands have a length less than about 1 nm. Thesecond ligands include at least one of pyridine, allylamine,methylamine, ethylamine, propylamine, butylamine, octylamine, andpyrrolidine. The second ligands bind to the outer surface of thenanocrystals with an affinity that is at least as large as an affinitywith which the first ligands bind to the outer surface of thenanocrystals. Exchanging the plurality of first ligands for a pluralityof second ligands includes precipitating the fabricated nanocrystals;washing the precipitated nanocrystals; and dispersing the washednanocrystals in a solution comprising the second ligands. Forming thefilm of ligand-exchanged nanocrystals includes solution-depositing theligand-exchanged nanocrystals onto a substrate. Solution-depositing theligand-exchanged nanocrystals includes spray-coating, dip-casting,drop-casting, evaporating, blade-casting, or spin-coating theligand-exchanged nanocrystals onto the substrate. Removing the secondligands includes volatilizing the second ligands during the step offusing the cores of adjacent nanocrystals. Volatilizing the secondligands causes a relatively small change in the volume of the film ofligand-exchanged nanocrystals. The volume changes by less than about 30%during ligand removal. Removing the second ligands includes performing achemical transformation of the ligands so as to remove them. Removingthe second ligands includes soaking the film of ligand-exchangednanocrystals in a solvent that dissociates the second ligands from theouter surface of the nanocrystals but which does not substantiallydissociate the nanocrystals of the film from each other. Removing thesecond ligands further includes maintaining the nanocrystals in asubstantially inert environment. The solvent includes methanol. Fusingthe cores of adjacent nanocrystals includes annealing the film ofligand-exchanged nanocrystals. Fusing the cores of adjacent nanocrystalsforms a substantially inorganic film having nanoscale features. Thenanoscale features have about the same size and shape of the individualnanocrystals before they were fused. Fusing the cores of adjacentnanocrystals to an extent that the nanocrystals substantially maintaintheir individual properties but are joined by regions through whichcurrent readily flows. A central absorption wavelength of thenanocrystals changes by less than about 10% when fused to one or moreadjacent nanocrystals. Fusing the cores of adjacent nanocrystalsincludes annealing the nanocrystals at a temperature of between 150° C.and 450° C. Fusing the cores of adjacent nanocrystals includes annealingthe film at a temperature of between room temperature and 150° C.Modifying the outer surfaces of the fused nanocrystals. Modifying theouter surfaces includes oxidizing the fused nanocrystals. Modifying theouter surfaces includes depositing a semiconductor shell on the fusednanocrystals. Modifying the outer surface includes forming one or moredefect states on the outer surfaces of the fused nanocrystals.

Under another aspect, a method of forming a device includes forming afilm of nanocrystals, the nanocrystals having a core and an outersurface, a plurality of ligands being attached to the outer surface, atleast a portion of the nanocrystals being in physical contact with atleast one adjacent nanocrystals; removing the ligands from at least aportion of the nanocrystals; annealing the film of nanocrystals so as tofuse the cores of the nanocrystals to the cores at least one adjacentnanocrystal and thus form an electrical network of fused nanocrystals;and providing first and second electrodes in spaced relation and inelectrical communication with first and second portions of theelectrical network of fused nanocrystals.

One or more embodiments include one or more of the following features:Also substituting a plurality of said ligands with a plurality ofshorter ligands. Substituting a plurality of said ligands with aplurality of shorter ligands decreases an effective distance between atleast one nanocrystals and at least one adjacent nanocrystal. Alsoaltering the composition of the outer surfaces of the nanocrystals. Alsocreating at least one defect state on the outer surface of at least someof the fused nanocrystals and not creating a defect state in the regionswhere one nanocrystal core is fused to another. Creating at least onedefect state on substantially each fused nanocrystal includes oxidizingthe electrical network of fused nanocrystals. The at least one defectstate includes at least one trap state for a hole during operation ofthe optical device. Forming the film of nanocrystals on the substrateincludes solution-depositing colloidal nanocrystals on the substrate.Solution-depositing colloidal nanocrystals includes spray-coating,dip-casting, drop-casting, evaporating, blade-casting, or spin-coatingthe nanocrystals onto the substrate. Providing first and secondelectrodes in spaced relation and in electrical communication with theelectrical network of fused nanocrystals includes forming the first andsecond electrodes on a substrate and subsequently performing steps(a)-(c). The first and second electrodes are spaced from each other bybetween about 0.2 and 2 μm. Providing the first and second electrodeshaving parallel orientation relative to each other. Providing the firstand second electrodes being interdigitated with one another. Providingfirst and second electrodes in spaced relation and in electricalcommunication with the electrical network of fused nanocrystals includesforming the first electrode on the substrate, subsequently performingsteps (a)-(c), and subsequently providing the second electrode over theelectrical network of fused nanocrystals. The second electrode includesat least one of aluminum, gold, platinum, silver, magnesium, copper,indium tin oxide (ITO), tin oxide, tungsten oxide, and combinations andlayer structures thereof. The second electrode is at least partiallyoptically transparent. The second electrode includes at least one of abandpass filter and a bandblock filter.

Under another aspect, a method of forming a nanocrystalline film from aplurality of nanocrystals, the nanocrystals having a core and an outersurface, a plurality of ligands being attached to the outer surface,includes forming a film of ligand-attached nanocrystals, wherein atleast a portion of the ligand-attached nanocrystals are adjacent atleast one other ligand-attached nanocrystal; removing the ligandsattached to the outer surfaces of the nanocrystals of the film ofligand-exchanged nanocrystals; and fusing the cores of adjacentnanocrystals so as to form an electrical network of fused nanocrystals.

One or more embodiments include one or more of the following features.The ligands each include a carbon chain between about 1-10 carbons long.The ligands have a length less than about 1 nm. Forming the film ofligand-attached nanocrystals includes solution-depositing theligand-exchanged nanocrystals onto a substrate. Solution-depositing theligand-exchanged nanocrystals includes spray-coating, dip-casting,drop-casting, evaporating, blade-casting, or spin-coating theligand-exchanged nanocrystals onto the substrate. Removing the ligandsincludes volatilizing the ligands during the step of fusing the cores ofadjacent nanocrystals. Removing the ligands includes soaking the film ofligand-attached nanocrystals in a solvent that dissociates the ligandsfrom the outer surface of the nanocrystals but which does notsubstantially dissociate the nanocrystals of the film from each other.Removing the ligands further includes maintaining the nanocrystals in asubstantially inert environment. Fusing the cores of adjacentnanocrystals includes annealing the film of ligand-attachednanocrystals. Fusing the cores of adjacent nanocrystals includesannealing the nanocrystals at a temperature between room temperature andabout 450° C. Also modifying the outer surfaces of the fusednanocrystals. Modifying the outer surfaces includes oxidizing the fusednanocrystals. Modifying the outer surfaces includes depositing asemiconductor shell on the fused nanocrystals. Modifying the outersurface includes forming one or more defect states on the outer surfacesof the fused nanocrystals.

Under another aspect, a film includes a network of fused nanocrystals,the nanocrystals having a core and an outer surface, wherein the core ofat least a portion of the fused nanocrystals is in direct physicalcontact and electrical communication with the core of at least oneadjacent fused nanocrystal, and wherein the film has substantially nodefect states in the regions where the cores of the nanocrystals arefused.

One or more embodiments include one or more of the following features.The outer surface of at least a portion of the fused nanocrystalsincludes a material of different composition than the core. The outersurface includes oxidized core material. The outer surface includessemiconductor material. The outer surface includes at least one defectstate. The film is substantially inorganic. The film is substantiallyfree of ligands on the outer surfaces of the fused nanocrystals. Thenetwork of fused nanocrystals defines a conductive electrical network.The network of fused nanocrystals has an electrical resistance of atleast about 25 k-Ohm/square. The network of fused nanocrystals has acarrier mobility of between about 0.001 and about 10 cm²/Vs. The networkof fused nanocrystals has a carrier mobility of between about 0.01 andabout 0.1 cm²/Vs. The network of fused nanocrystals is opticallysensitive. The network of fused nanocrystals has a substantially linearresponsivity to irradiation in at least a portion of the electromagneticspectrum. The film is disposed on a substrate. The substrate is flexibleand formed in a non-planar shape. The substrate includes an integratedcircuit, at least some components of which are in electricalcommunication with the film. The substrate includes at least one of asemiconducting organic molecule, a semiconducting polymer, and acrystalline semiconductor. The film has an electrical resistance of atleast about 25 k-Ohm/square. The network of fused nanocrystals includesfused nanocrystals of different compositions. The network of fusednanocrystals includes fused nanocrystals of different sizes. The fusednanocrystals are substantially monodisperse. The fused nanocrystalsinclude at least one of PbS, InAs, InP, PbSe, CdS, CdSe,In_(x)Ga_(1-x)As, (Cd—Hg)Te, ZnSe(PbS), ZnS(CdSe), ZnSe(CdS), PbO(PbS),and PbSO₄(PbS). The film has an optical response to irradiation in atleast one of the infrared, ultraviolet, x-ray, and visible regions ofthe electromagnetic spectrum. The optical response of the film isrelated to a size of the fused nanocrystals in the film. The fusednanocrystals have individual properties that vary by less than about 10%from the individual properties of unfused nanocrystals having the samesize, shape, and composition as the fused nanocrystals.

Under another aspect, a device includes a film comprising a network offused nanocrystals, the nanocrystals having a core and an outer surface,wherein the core of at least a portion of the fused nanocrystals is indirect physical contact and electrical communication with the core of atleast one adjacent fused nanocrystal, and wherein the film hassubstantially no defect states in the regions where the cores of thenanocrystals are fused; and first and second electrodes in spacedrelation and in electrical communication with first and second portionsof the network of fused nanocrystals.

One or more embodiments include one or more of the following features.The film is substantially free of ligands attached to the outer surfaceof the fused nanocrystals. The outer surfaces of the fused nanocrystalsinclude a material having a different composition than the core. Theouter surfaces include at least one defect state. The at least onedefect state includes at least one trap state for a hole duringoperation of the optical device. The outer surfaces include asemiconductor material. The outer surfaces include oxidized corematerial. The electrical network of fused nanocrystals provides aplurality of relatively low-resistance electrical paths from the firstelectrode to the second electrode. The film has an electrical resistanceof at least about 25 k-Ohm/square. The electrical resistance of the filmchanges in response to irradiation by light. The electrical network offused nanocrystals provides a plurality of electrical paths from thefirst electrode to the second electrode and at least some of thoseelectrical paths undergo a change in electrical resistance in responseto incident light. The film has a carrier mobility of between about0.001 cm²/Vs and about 10 cm²/Vs. The film has a carrier mobility ofbetween about 0.01 and cm²/Vs and about 0.1 cm²/Vs. The fusednanocrystals are substantially monodisperse. The fused nanocrystalsinclude a plurality of a first type of fused nanocrystals and aplurality of a second type of fused nanocrystals. The core ofsubstantially each of the first type of fused nanocrystals is in directphysical contact and electrical communication with the core of anotherof the first type of fused nanocrystals. The core of substantially eachof the second type of fused nanocrystals is in direct physical andelectrical communication with the core of another of the second type offused nanocrystals. Each fused nanocrystal is of a size and compositionto absorb at least one of infrared radiation, x-ray radiation,ultraviolet radiation, and visible radiation. The first and secondelectrodes are disposed on a substrate with the film therebetween. Thefirst and second electrodes are substantially parallel to each other.The first and second electrodes are interdigitated. The first electrodeis disposed on a substrate, the film is over the first electrode, andthe second electrode is over the film. The first and second electrodesare spaced by about 0.2 μm to about 2 μm from each other.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying drawings. The drawings are notnecessarily to scale. For clarity and conciseness, certain features ofthe invention may be exaggerated and shown in schematic form. In thedrawing:

FIG. 1 shows a schematic of a known quantum dot nanocrystal.

FIG. 2 shows a two-dimensional schematic of a layer of fused quantumdots.

FIG. 3A shows an optical micrograph of a light sensitive layer formed onan electronic read-out chip.

FIG. 3B shows side view of an optical device which includes anintegrated circuit with an array of electrodes located on the topsurface thereof.

FIG. 4A is a side view of a portion of an optical device configured in avertical sandwich structure.

FIG. 4B is a side view of a portion of an optical device configured in alateral planar structure.

FIG. 4C is a plan view of a portion of an optical device configured in alateral interdigitated structure.

FIG. 5 shows an overview of steps in a method of making a QD opticaldevice with enhanced gain and sensitivity.

FIG. 6A shows absorption spectra and TEM images of lead sulfide QDs asligands are exchanged from oleic acid to primary butylamine.

FIG. 6B shows absorption spectra and TEM images of lead sulfide QDs asligands are exchanged from oleic acid to primary butylamine.

FIG. 6C shows FTIR spectra of lead sulfide QDs after ligands areexchanged to primary butylamine.

FIG. 6D shows FTIR spectra of lead sulfide QDs with butylamine ligandsbefore and after methanol wash.

FIG. 6E shows XPS data of lead sulfide QDs through various processingstages.

FIG. 6F shows FTIR data of lead sulfide QDs precipitated in inert and inoxidizing conditions.

FIG. 7A shows a device structure (inset) and plots of I-V characteristicfor 100 nm and 500 nm QD layer devices.

FIG. 7B shows the responsivity of a variety of QD layer devices.

FIG. 7C shows the dark current density of a variety of QD layer devices.

FIG. 7D shows the measured noise current as a function of measured darkcurrent for a variety of QD layer devices.

FIG. 7E shows the normalized detectivity of a variety of QD layerdevices.

FIG. 7F shows spectra of responsivity and normalized detectivity of a QDlayer device.

FIG. 7G shows the electrical frequency response of a QD layer device.

FIG. 7H shows the temporal response of a QD layer device.

FIG. 8 shows spectral dependence of responsivity and normalizeddetectivity D* at various levels of applied bias under 5 Hz opticalmodulation and 0.25 nW incident optical power.

FIG. 9 shows the frequency dependence of responsivity and nonnalizeddetectivity at different levels of applied bias at 975 nm and 0.25 nW ofincident optical power.

FIG. 10 shows the noise equivalent exposure of a QD layer device ascompared with conventional Si CCD and CMOS sensor devices.

DETAILED DESCRIPTION

Overview

The present invention provides quantum dot (QD) devices and methods ofmaking devices. Many embodiments are optical devices with enhanced gainand sensitivity, and which can be used in optical and infrared (IR)imaging applications, photovoltaic applications, among otherapplications. The term “quantum dot” or “QD” is used interchangeablyherein with the term “nanocrystal,” and it should be understood that thepresent invention is not limited exclusively to quantum dots but ratherto any “nanoscale” crystalline material.

Some embodiments of the QD optical devices are single image sensor chipsthat have a plurality of pixels, each of which includes a QD layer thatis light sensitive, e.g., optically active, and at least two electrodesin electrical communication with the QD layer. The current and/orvoltage between the electrodes is related to the amount of lightreceived by the QD layer. Specifically, photons absorbed by the QD layergenerate electron-hole pairs, generating a current and/or voltage. Bydetermining the current and/or voltage for each pixel, the image acrossthe chip can be reconstructed. The image sensor chips have a highsensitivity, which can be beneficial in low-light applications; a widedynamic range allowing for excellent image detail; and a small pixelsize, which to a large extent is limited by currently available CMOStechniques such as lithography. The responsivity of the sensor chips todifferent optical wavelengths is also tunable by changing the size ofthe QDs in the device, by taking advantage of the quantum size effectsin QDs. The pixels can be made as small as 1 micron square or less.

In many embodiments, the optically sensitive QD layer includes aplurality of QDs that have been specially processed to give the layer anenhanced gain and sensitivity as compared with conventionalsilicon-based layers as well as other kinds of QD layers, such as thosedescribed in the incorporated patent references. Specifically, aplurality of QDs are fabricated using well-known techniques, andtypically include a core as well as an outer surface that includes aplurality of ligands. The ligands are exchanged for shorter, volatileligands, and then the ligand-exchanged QDs are solution-deposited onto asubstrate to form a QD precursor layer. The substrate itself may includeone or more electrodes, or the electrodes may be deposited in a laterstep. Subsequently, the short ligands are removed from the QD precursorlayer. This brings the QDs in the QD precursor layer into very closecontact, so that at least some of the QDs contact their neighbors. Thiscontact between QDs may be referred to as “necking” Subsequently, thenecked QD layer is annealed, which fuses the necked QDs together. The QDprecursor layer is typically maintained in an inert atmosphere afterligand removal, so that the outer surfaces of the individual QDs do notoxidize until annealing is complete.

While two given fused QDs in the annealed QD layer retain a largeportion of their original shape, and thus remain individuallyrecognizable, after annealing the QDs are no longer physically distinctfrom each other. Instead, the cores of the QDs together form acontinuous electrical path. Thus, if many adjacent QDs neck, duringannealing those necked QDs fuse to form an electrical network with aphysical extent that is substantially greater than that of theindividual QDs, and through which current will readily flow. Forexample, the fused QD film may have a macroscopic extent, though the QDsthemselves are nanoscopic. In some embodiments, the finished QD layerafter ligand removal, necking, and annealing, can essentially beconsidered a continuous inorganic film having nanoscale features. Thegeneral shapes of the individual QDs may still be recognizable, buttheir cores form a continuous electrical network that is mechanicallyrobust. For example, a micrograph of the finished QD layer would showthe general shape and size of the individual QDs from which the layer isformed, as well as robust joints between many adjacent QDs.

In many embodiments, the fused QD layer is subsequently processed tomodify its outer surfaces. For example, a material such as asemiconductor shell can be coated on the outer surfaces of the fusedquantum dots. Or, for example, defect states can be formed on theexposed outer surfaces of the QDs, e.g., by oxidizing the fused QDslayer. These defect states effectively trap holes generated by photons,so that they recombine with electrons far less readily and thus greatlyenhance the amount of current that a given photon generates in thefinished QD layer, i.e., greatly enhance the photoconductive gain of thedevice. The fused QD cores, and the juncture between them, willgenerally not have defect states, so current will flow readily betweenthem, in certain embodiments.

FIG. 2 shows a two-dimensional representation of a portion of a QDlayer. The layer includes a continuous network of fused QD cores 20,having outer surfaces 21 that are a different composition than that inthe core, e.g., oxidized core material, or a different kind ofsemiconductor. The individual QD cores in the film are in intimatecontact, but continue to exhibit many of the normal properties ofindividual quantum dots. For example, a lone (unfused) quantum dot has awell-characterized central absorbance wavelength that arises fromquantum effects related to its tiny size, e.g., 1-10 nm. The centralabsorbance wavelength of the fused QDs in the film is not significantlyshifted from their central absorbance wavelength before fusing. Forexample, their central absorbance wavelength may change by about 10% orless when fused. Thus, the QDs in the film retain their quantum effects,despite the fact that they may be an integral part of a macroscopicstructure.

Current is not generally though of as “flowing” through a lone (unfused)QD; instead, electrons simply occupy well-known quantum energy states inthe CD core. If two lone (unfused) QDs are brought near each other,current can “flow” between them by electron “hopping” between the QDs,which has a well-known dynamic. In contrast, current does readily flowbetween fused QD cores, even though the cores themselves generallyretain their quantum energy states. Because the cores are in contact,electrons can easily move between them. This aspect of QD fusingtypically provides a carrier mobility of between about 0.001-10 cm²/Vs,in some embodiments between about 0.01-0.1 cm²/Vs, for example greaterthan 0.01 cm²/Vs, while at the same time not fusing the QDs to an extentthat they lose their “identity,” namely their individual characteristicsthat provide quantum confinement. A film of fused QDs typically alsoexhibits a a relatively low electrical resistance pathway, e.g., havinga resistance above about 25 k-Ohm/square It is also possible to“overfuse” QDs, in which case they no longer exhibit many of the normalproperties of individual quantum dots. In the overfused case, the coresof the QDs do not generally have their own quantum energy levels, butthe energy levels are instead distributed over multiple QD cores. Thisresults in a film with a very low electrical resistance, e.g., less thanabout 25 k-Ohm, but which in many ways is effectively a bulksemiconductor material. “Overfused” QDs can also be recognizedexperimentally as a relatively large shift (e.g., greater than about10%) in a shift to the red (longer wavelengths) in their absorptionand/or emission spectra.

In certain embodiments the QD layer is exceptionally light sensitive.This sensitivity is particularly useful for low-light imagingapplications. At the same time, the gain of the device can bedynamically adjusted so that the device will not “saturate,” that is,additional photons continue to provide additional useful information.Tuning of gain can be conveniently achieved by changing the voltagebias, and thus the resultant electric field, across a given device,e.g., a pixel. As discussed in greater detail below, photoconductivegain, and correspondingly the responsivity in A/W, varies approximatelylinearly with bias and field. Thus, in a given device, a bias of about0.1 V may result in a gain of about 10, while a bias of about 10 V mayresult in a gain of about 100.

Some embodiments of QD devices include a QD layer and a custom-designedor pre-fabricated CCD or CMOS electronic read-out integrated circuit.CCD and CMOS electronic read-out circuits are readily commerciallyavailable at low cost. The QD layer is then formed directly onto thecustom-designed or pre-fabricated CCD or CMOS electronic read-outintegrated circuit. The QD layer may additionally be patterned so thatit forms individual islands. Wherever the QD layer overlies the circuit,it continuously overlaps and contacts at least some of the features ofthe circuit. If the QD layer overlies three-dimensional features of thecircuit, the QD layer conforms to those features. In other words, thereis a substantially contiguous interface between the QD layer and theunderlying CCD or CMOS electronic read-out integrated circuit. One ormore electrodes in the CCD or CMOS circuit contact the QD layer and arecapable of relaying information about the QD layer, e.g., the amount oflight on it, to a readout circuit. The QD layer can be provided in acontinuous manner to cover the entire underlying circuit, such as areadout circuit, or patterned. If in a continuous manner, the fillfactor can approach about 100%, which is much greater than known CMOSpixels; with patterning, the fill factor is reduced, but can still bemuch greater than a typical 35% for a CMOS sensor.

In many embodiments, the QD optical devices are readily fabricated usingstandard CMOS techniques. For example, a layer of QDs can besolution-coated onto a prefabricated CCD or CMOS electronic read-outcircuit using, e.g., spin-coating, which is a standard CMOS process, andoptionally further processed with other CMOS-compatible techniques toprovide the final QD layer for use in the device. Details of QDdeposition and further processing are provided below. Because the QDlayer need not require exotic or difficult techniques to fabricate, butcan instead be made using standard CMOS processes, the QD opticaldevices can be made in high volumes, and with no significant increase incapital cost (other than materials) over current CMOS process steps.

Individual features and embodiments of QD devices, and methods of makingsame, will now be described in greater detail.

Electronic Read-Out Integrated Circuit

FIG. 3A shows an optical micrograph of a light sensitive layer formed ona commercially available electronic read-out integrated circuit (ROIC)chip. In many embodiments, the light sensitive layer includes aplurality of QDs, as described in greater detail below. The lightsensitive layer, e.g., the QD layer, overlays and conforms to thefeatures of the underlying chip. As can be seen in FIG. 3A, theelectronic read-out chip, e.g., a CCD or CMOS integrated circuit,includes a two-dimensional array of rows 310 and columns of electrodes320. The electronic readout chip also includes a two-dimensional arrayof square electrode pads 300 which together with the overlying QD layerand other circuitry form a two-dimensional array of pixels. The rowelectrodes 310 and column electrodes 320 allow each pixel (includingsquare electrode pad 300 and overlying QD layer) to be electronicallyread-out by a read-out circuit (not shown) that is in electricalcommunication with the electrodes. The resulting sequence of informationfrom the ROIC at the read-out circuit corresponds to an image, e.g., theintensity of light on the different regions of the chip during anexposure period, e.g., a frame. The local optical intensity on the chipis related to a current flow and/or voltage bias read or measured by theread-out circuit.

FIG. 3B shows a schematic cross-section of the imaging system shown inFIG. 3A. The imaging system includes a read-out structure that includesa substrate 12, an optically sensitive layer 38, e.g., QD layer 38 and atransparent electrode 36. Substrate 32 includes a read-out integratedcircuit (ROIC) having a top surface with an array of pixel electrodes 34located in the top surface thereof with counter-electrodes 36 locatedoutside the array, i.e., transparent electrode 36 overlaying QD layer38. Electrodes 34 shown in FIG. 3B correspond to square electrode pads300 shown in FIG. 3A. The array of electrodes 34, which together formfocal plane array 30, provide voltages for biasing and currentcollection from a given pixel 34 of the array, and convey signals fromthe array to an input/output electrode 32 (connection not shown).Optically sensitive layer 38, e.g., QD layer, is formed on the topsurface of the integrated circuit. More specifically, QD layer 38overlays the array of pixel electrodes 34 on the top surface of theintegrated circuit. The optically sensitive layer 38 defines an array ofimaging pixels for collecting light incident thereon.

In the imaging system of FIG. 3B, QD layer 38 is monolithicallyintegrated directly onto the electronic read-out chip. In contrast, aspreviously mentioned, existing imaging systems are often made byseparately fabricating 1) the read-out integrated circuit and 2) thesensitizing semiconductor array, and subsequently assembling the two,e.g., using a process such as microbump bonding.

Referring now to FIG. 4A, there is shown at 40 a side view of a basicoptical device structure, which in certain embodiments can be used as anindividual pixel in the completed integrated array shown in FIGS. 3A-3B.Device 40 includes substrate 42, which may be glass or other compatiblesubstrate; contact/electrode 44; optically sensitive layer, e.g., QDlayer 48; and at least partially transparent contact 45 that overlaysthe QD layer. Contacts 44 and 45 may include, e.g., aluminum, gold,platinum, silver, magnesium, copper, indium tin oxide (ITO), tin oxide,tungsten oxide, and combinations and layer structures thereof, and mayinclude band-pass or band-block filters that selectively transmit orattenuate particular regions of the spectrum which are appropriate tothe end use of the device. The device has an overall “vertical sandwich”architecture, where different components of the device generally overlayother components. In operation, the amount of current that flows and/orthe voltage between contact 45 and contact 44 is related to a number ofphotons received by QD layer 48. In operation, current generally flowsin the vertical direction. The embodiment shown in FIG. 4A may alsoinclude one or more additional optional layers for electron/holeinjection and/or blocking The layer(s) allows at least one carrier to betransported, or blocked, from an electrode into the QD layer. Examplesof suitable layers include a QD layer including QDs of a different sizeand/or composition, semiconducting polymers, and semiconductors such asITO and Si.

Referring now to FIG. 4B, there is shown at 40′ a side view of a basicdevice structure which has a different configuration than each pixel inthe completed integrated array shown in FIGS. 3A-3B, but which could beused to form a similarly functioning optical device. The configurationin FIG. 4B corresponds to a lateral planar structure in which theoptically sensitive layer 48′ is deposited across two spacedcontacts/electrodes 44′ and 46. Contacts 44 and 46 are deposited on asubstrate, e.g., glass substrate 42′. The integrated circuit, includingcontacts 44′, 46, and substrate 42′ may include any appropriate systemwith which the optically sensitive material is compatible (e.g., Si,glass, plastic, etc.). Contacts 44′ and 46 may include aluminum, gold,platinum, silver, magnesium, copper, indium tin oxide (ITO), tin oxide,tungsten oxide, or combinations or layer structures thereof. The devicehas an overall “lateral planar” architecture, where at least some of thecomponents of the device are generally laterally displaced from othercomponents, forming a planar electrode structure. In operation, theamount of current that flows and/or the voltage between contact 44′ andcontact 46 is related to a number of photons received by the QD layer48′. In operation, current generally flows in the lateral direction.

FIG. 4C shows a plan view of another basic device structure 40″ thatincludes interdigitated electrodes, and which also can be used to forman optical device. The materials may be selected from those providedabove regarding FIGS. 4A-4B.

Each basic device 40, 40′, and 40″ as shown in FIGS. 4A-4C, among otherpossible architectures, can be thought of as representing a singledevice or an element in a larger device, such as in a linear array or atwo-dimensional array. The basic devices can be used in many kinds ofdevices, such as detection and signal processing, as discussed above, aswell as in emission and photovoltaic devices. Not all embodiments needbe optical devices. Many QD layers have optical characteristics that canbe useful for optical devices such as image sensors useful in one ormore of the x-ray, ultraviolet, visible, and infrared parts of thespectrum; optical spectrometers including multispectral andhyperspectral; communications photodetecting optical receivers as wellas free-space optical interconnection photoreceivers; and environmentalsensors; Some QD layers also have electrical characteristics that may beuseful for other kinds of devices, such as transistors used in signalprocessing, computing, power conversion, and communications.

In one embodiment, the underlying electrodes on the integrated circuitdefine imaging pixels in an imaging device. The QD layers formed on theelectrodes supply optical-to-electrical conversion of incident light.

In another embodiment, in addition to the definition of pixels viaelectrodes on the integrated circuit, further patterning of theoptically sensitive layers, e.g., QD layers, provides further definitionof pixels, including of which pixel is read by which electrodes on theintegrated circuit. This patterning may also be accomplished withwell-known CMOS techniques such as photolithography. Other optionsinclude self-assembly of QD layers onto pre-patterned metal layers, suchas Au, to which the QDs and/or their ligands have a known affinity.Patterning may also be achieved by depositing a conformal QD layer ontoa topologically-variable surface, e.g., including “hills” (protrusions)and “valleys” (trenches) and subsequently planarizing the QD film toremove material accumulated on the “hills” while preserving that in the“valleys.”

Further layers may be included in the layers atop the structure, such aselectrical layers for making electrical contact (e.g. an at leastpartially transparent contact such as indium tin oxide, tin oxide,tungsten oxide, aluminum, gold, platinum, silver, magnesium, copper, orcombinations or layer structures thereof), antireflection coatings (e.g.a series of dielectric layers), or the formation of a microcavity (e.g.two mirrors, at least one formed using nonabsorbing dielectric layers.),encapsulation (e.g. an epoxy or other material to protect variousmaterials from environmental oxygen or humidity), or optical filtering(e.g. to allow visible light to pass and infrared light not to, orvice-versa.)

The integrated circuit may include one or more semiconducting materials,such as, but not limited to silicon, silicon-on-insulator,silicon-germanium layers grown on a substrate, indium phosphide, indiumgallium arsenide, gallium arsenide, or semiconducting polymers such asMEH-PPV, P3OT, and P3HT. The integrated circuit may also include one ormore semiconducting organic molecules, non-limiting examples beingend-substituted thiophene oligomers (e.g. alpha,w-dihexyl hexathiophene(DH6T)) and pentacene. Polymers and organic molecules can be useful as asubstrate in the QD devices because they may be flexible, and thus allow“bendable” and “conformable” devices to be made that are thusnon-planar.

Other appropriate substrates may include, e.g., plastic and glass.

Optically Sensitive Layer

The optically sensitive layer includes a material that is opticallysensitive in any one or more of the infrared, visible and ultravioletregion of the electromagnetic spectrum. As discussed above, in manyembodiments the optically sensitive layer includes one or more types ofquantum dot nanocrystals (QDs), which may be fused together.

In some embodiments, the optically sensitive layer includes acombination of two or more types of QDs, each including a distinctsemiconductor material and/or having distinct properties. The differenttypes of QDs may be separately synthesized and mixed prior to beingapplied to the surface of the integrated circuit or they may besynthesized in a ‘one pot’ synthesis—i.e. in a single vessel.

In some embodiments, the optically sensitive layer includes an opticallysensitive semiconducting polymer such as, but not limited to MEH-PPV,P3OT and P3HT. In other embodiments the optically sensitive layerincludes a polymer-QD mixture having one or more types of QDs thatsensitive to different parts of the electromagnetic spectrum.

A. Quantum Dot Nanocrystals

In many embodiments, the QDs are fabricated using known techniques, butin substantially inert, anhydrous environments, e.g., environments thatare substantially free of water and oxygen. Syntheses may be performedusing Schlenk line methods in which ambient gases such as oxygen andwater in the air are excluded from the system, and the syntheses areinstead performed in the presence of substantially inert gases such asnitrogen and/or argon, or in a vacuum.

In some embodiments, the QDs include any one or combination of PbS,InAs, InP, PbSe, CdS, CdSe, ternary semiconductors, and a core-shelltype semiconductors in which the shell is one type of semiconductor andthe core is another type of semiconductor. For example, the ternary QDsmay be In_(x)Ga_(1-x)As nanocrystals or (Cd—Hg)Te nanocrystals. Forexample, the core-shell quantum dot nanocrystals may be ZnSe(PbS),ZnS(CdSe), ZnSe(CdS), PbO(PbS), or PbSO₄(PbS).

In some embodiments, before depositing the QD precursor layer on theintegrated circuit or substrate, the QDs are ligand exchanged tosubstitute the as-fabricated ligands with pre-selected ligands, e.g.,ligands that are considerably shorter than the as-fabricated ligands.The pre-selected ligands are selected to be sufficiently short to enablecloser packing of the QDs in the precursor layer. Closer packing allowsthe QDs to fuse together in a subsequent step, thereby greatlyincreasing the electrical conductivity between the QDs. The preselectedligands may also be selected to be relatively volatile, so that they canbe vaporized during a subsequent step to provide a film consistingmainly of QDs and being substantially free of ligands. This allows theQDs to get much closer to each other, which may enhance the conductivityin the final device. For example, the QDs may be fabricated with a firstset of ligands with carbon chains that are more than 10 carbons long;the first set of ligands is then substituted with a second set ofligands with carbon chains that are between 1-10 carbons long. In somecircumstances, the ligands of the second set of ligands is less thanabout 1 nm long. This can bring the QDs closer, e.g., more than 50%closer, more than 75% closer, or even more than 90% closer, than theycould get before ligand exchange. The second set of ligands maygenerally have an affinity for attachment to the QDs that is at leastcompetitive with the affinity of the first set of ligands to attach tothe QDs, otherwise the first set of ligands may not sufficientlyexchange with the first set of ligands. The second set of ligands mayalso generally have an affinity for attachment to the QDs which allowsthem to be removed during a later step. This affinity is related to theend functional group on the ligand, which is illustrated in FIG. 1.Amines, thiols, carboxylates, and sulfones, among other end functionalgroups, many of which will have free electron pairs, are generallysuitable for use in the second (pre-selected) set of ligands.

In some embodiments, the ligand exchange involves precipitating theas-synthesized QDs from their original solution, washing, andredispersing in a liquid that will dissolve and thus dissociate theoriginal ligands from the outer surfaces of the QDs, and which either isor contains the ligands to be substituted onto the QDs. In someembodiments the liquid is or includes primary, secondary, ortertiary-butylamine, pyridine, allylamine, methylamine, ethylamine,propylamine, octylamine, or pyrrolidine or a combination of theseorganic solvents, which substitute the ligands previously on the QDs. Inother embodiments, the liquid is or includes pyridine, which substitutesthe ligands previously on the QDs. Leaving the QDs in this liquid forbetween 24 and 120 hours either at room temperature or at an elevatedtemperature is generally sufficient for ligand exchange, although insome circumstances longer or shorter times will be sufficient. In anillustrative example, the ligand exchange process was performed under aninert atmosphere to prevent the QDs from oxidation. QDs having oleateligands and dissolved in methanol were precipitated, dried, andredispersed in n-butylamine at a concentration of 100 mg/ml(nanocrystals by weight/butylamine by volume). The solution was left for3 days under inert conditions. The oleate ligands had a length of about2.5 nm, and the exchanged butylamine ligands had a length of about 0.6nm, bringing the QDs to about 25% of their original distance from eachother.

In some embodiments, two or more types of QDs are separately fabricatedin coordinating solvents. Each kind of QD is then precipitated, washed,and dispersed in a liquid that is or contains the ligands to besubstituted onto the QDs. This exchanges the ligands on the two or moretypes of QDs as discussed above. Then the two types of QDs are mixed insolution to create a heterogeneous QD mixture, which is spin-cast orotherwise deposited as thin films on a substrate to form a heterogeneousQD precursor layer. The order in the heterogeneous QD precursor layer iscontrolled through separate selection of QD size and ligand for eachtype of QD and additional treatment with solvents and heating.

Examples of ligands include amine-terminated ligands,carboxyl-terminated ligands, phosphine-terminated ligands and polymericligands. The amine-terminated ligands may include any one or combinationof pyridine, allylamine, methylamine, ethylamine, propylamine,butylamine, octylamine, and pyrrolidine. The carboxyl-terminated ligandsmay include any one or combination of oleic acid, stearic, capric andcaproic acid. The phosphine-terminated ligands may include guanosinetriphosphate. The ligand may be one or more of DNA, an oligonucleotide,a polymer such as polythiophene or MEH-PPV, or an oligomer such asoligothiophene. As mentioned above, it can be useful to substitute shortand volatile ligands, e.g., pyridine, allylamine, methylamine,ethylamine, propylamine, butylamine, octylamine, or pyrrolidine, ontothe QDs so that the QDs can be brought into closer proximity in latersteps.

B. Forming Precursor OD Layer on Integrated Circuit

After the QDs are fabricated and ligand-exhanged, e.g., as describedabove, they may be deposited onto a substrate such as an integratedcircuit. This forms a “QD precursor layer,” which may be subsequentlyprocessed to form a finished QD layer for use in a device.

The QD precursor layer may be formed by solution-depositing it directlyon the surface of a read-out integrated circuit or other substrate, forexample using spray-coating, dip-casting, drop-casting, evaporating, orblade-casting. Another method of depositing the QD precursor layer isspin coating the QD precursor layer, which once spin-coated onto thesurface may be further processed to form the optically sensitive QDlayer as described below. In many embodiments, the QD layer has athickness selected to absorb most or even substantially all of the lightincident on it, in the wavelength region the device is intended tooperate in. Typically this thickness will range between about 50 nm and2 μm, though thinner or thicker films can be used according to thedesired functionality of the device. Spin-coating can allow the processof covering circuitry with a QD layer to be performed at lowertemperatures without vacuum processing and alignment and bonding issues.

C. Ligand Removal and Annealing of OD Precursor Layer

After forming the QD precursor layer, the QDs may be fused together toproduce a QD film with enhanced optical and electrical characteristics,and which is suitable for use in a finished electronic or optoelectronicdevice.

In one embodiment, at least a portion of the QDs in the QD precursorlayer are fused by annealing the layer at temperatures up to about 450°C., or between about 150° C. and 450° C. In other embodiments, the layeris treated at lower temperatures, for example between about roomtemperature up to about 150° C., or up to about 100° C., or up to about80° C. In some embodiments, the QD precursor layer is not heatedsubstantially above ambient (room) temperature. As mentioned above, thestep of fusing brings the cores of adjacent QDs into direct physical andelectrical contact. It is also possible to “overfuse” the QDs, in whichcase they may lose their individual characteristics and appear more likea bulk semiconductor material. It is desirable to prevent suchoverfusing through the parameters chosen for annealing or throughmonitoring to prevent an overfused condition. The annealing step willtypically be performed in a vacuum or in an otherwise anhydrousenvironment to prevent the development of defect states (e.g.,oxidation) on the outer surfaces of the QDs before the cores of the QDsfuse together. This way, there will be substantially no defect states inthe regions where the QDs are joined together, but these regions insteadwill have a substantially homogeneous composition and crystallinestructure. In other embodiments the fusing step may be performed in anoxygen-rich environment, or an oxygen environment in which the partialpressure of oxygen is regulated.

The ligands in the QD precursor layer are also typically removed, eitherbefore or concurrently with the fusing step. For example, if the ligandsin the QD precursor layer are volatile, they may easily be removedduring annealing because they will simply volatilize from the heat. Or,for example, if the ligands in the QD precursor layer are not volatile,they can be removed from the QD precursor layer by soaking the layer ina solvent that dissolves and thus dissociates the ligands from the QDsbut which does not generally disrupt the arrangement of QDs in the QDlayer. In general, it is preferable that removing the ligands does notsignificantly change the volume of the QD layer, e.g., by less thanabout 30%; a large volume change may crack or otherwise damage thefinished QD film.

D. Creation of Defect States on Outer Surfaces of Fused ODs

In many embodiments, particularly those suitable for opticalapplications, defect states are created on the outer surfaces of thefused QDs. By “defect state” it is meant a disruption in the otherwisesubstantially homogeneous crystal structure of the QD, for example, thepresence of a dislocation or a foreign atom in the crystal lattice. Inmany cases this defect state will exist on the outer surface of the QDs.A defect state can be created by, e.g., oxidizing the QDs after fusingand ligand removal. During operation, if an electron-hole pair isgenerated within the QD film, one or more holes may be trapped by thedefect state; this will preclude rapid recombination of holes withelectrons, which will then allow the electrons to flow for a much longertime through the film. This can positively affect photoconductive gain,among other things.

In general, the outer surface of the fused QDs can be coated orotherwise treated so it has a different composition than the cores ofthe fused QDs. For example, the outer surface can include asemiconductor or insulator shell.

E. Summary of Steps in Fabricating OD Layer

FIG. 5 shows a flow chart of steps in a method for creating variousembodiments of QD layers for use in optical devices.

First, the QDs are fabricated (500), e.g., using well-known techniques.The QDs will typically include a plurality of relatively long ligandsattached to their outer surfaces.

Then, the QDs are ligand-exchanged (510), e.g., by substituting shorterligands for those used during fabrication of the QDs. This step mayallow the QDs to pack more closely in subsequent processing steps.

Then, the QDs are deposited on a suitable substrate (520), e.g., on anelectronic read-out integrated circuit. This step may be accomplishedwith various solution-based methods, many of which are compatible withstandard CMOS processes such as spin-coating.

Then, the precursor layer is washed to remove the ligands on the QDs,and to cause necking (i.e. touching) between at least some adjacent QDs(540).

Then, the necked QD layer is annealed, which fuses necked QDs together(540).

Then, defect states are created in the fused QD layer (550), e.g., byoxidizing the layer.

In general, when fabricating a device intended to have multiple pixels,the QD layer may then optionally be patterned, e.g., usingphotolithography, to separate the continuous layer into a plurality ofpixels.

The resulting QD layer can be incorporated into devices such as thosedescribed herein.

Examples

An exemplary photoconductive detector was made using a single layer ofPbS QD nanocrystals spin-cast directly from a chloroform solution ontoan interdigitated electrode array. The device structure is illustratedin FIG. 7A, and is analogous to the basic device of FIG. 4B. Theparallel gold electrodes are supported by a glass substrate and have aheight, width, and separation of 100 nm, 3 mm, 5 μm, respectively. Thethickness of the QD layer was controlled through the concentration ofthe chloroform-QD solution and the spin-casting parameters. In studiescarried out by the inventors the thickness ranged from 100 nm up to 500nm.

The treatment of the surfaces of the QDs was an important determinant ofphotodetector performance. Devices made directly from QDs capped witholeic acid, as synthesized through an organometallic route, did notexhibit any measurable conductance, as the 2 nm-long oleate ligandinhibits carrier transport among QDs. A post-synthesis ligand exchangewas therefore used to replace the as-synthesized oleate ligands withmuch shorter butylamine ligands. To this end, the QDs were redispersedin butylamine for a period of three days. Butylamine is afour-carbon-atom chain with an amine head as the functional group toattach to the QD surface. The ligand exchange was monitored for blueshift in QD absorption resulting from a decrease in QD effectivediameter as ligands remove Pb atoms during exchange.

FIG. 6A shows absorption spectra and TEM images of lead sulphide QDnanocrystals as the ligands are exchanged from oleic acid to primarybutylamine. The TEM images illustrate the dramatic decrease of inter-QDspacing following ligand exchange and nonsolvent treatment. Theabsorption spectrum shifts steadily to the blue with increasing exchangetime. When the shift is less than that associated with the removal of amonolayer of Pb atoms (roughly 170 nm), the size distribution remainsroughly constant. After this point the polydispersity increases. In theexample provided, the best device performance was obtained using QDnanocrystals shifted by ˜170 nm.

The QDs were precipitated, washed using a nonsolvent, redispersed inCHCl₃, and treated again using a nonsolvent (“nonsolvent” refers to amaterial that is not a solvent for the nanocrystals, but that may beasolvent for the ligands). The impact of ligand exchange and nonsolventtreatment on QDs is illustrated in the transmission electron micrographsof FIG. 6A. The as-grown (untreated) QD nanocrystals show well-orderedpatterns with interdot spacing determined by ligand length. Exchangedand washed QDs exhibit a drastic reduction in interdot spacing andpreferential formation of clusters instead of well-ordered arrays. Priorto treatment, the nanocrystal films can be redispersed using organicsolvents, while after treatment, nanocrystal films can no longer bereadily redispersed.

The combination of ligand exchange, nonsolvent treatment, and thermalprocessing at temperatures such as up to about 150° C. (typically) andpotentially as high as 450° C., removes at least a portion of the QDs'ligands, and enables the QDs to fuse, providing mechanically robustfilms with vastly increased electrical conductivity, as reported below.

FIG. 6B shows the absorbance spectra of QD nanocrystals before ligandexchange (oleate-capped), after ligand exchange (butylamine-capped), andfollowing soaking in methanol for 2 hours to remove the butylamineligands. The progressive blueshift across these treatments is consistentwith surface modification following exchange and partial surfaceoxidation (also confirmed by XPS and

FTIR). The inset of FIG. 6B shows TEM micrographs of the nanocrystalsbefore and after ligand exchange. The reduction in inter-particledistance is attributed to the replacement of the oleic acid ligands withbutylamine ligands.

FIG. 6C shows the FTIR spectra of the neat solvent n-butylamine, theneat solvent chloroform, and n-butylamine-exchanged QDs dispersed inchloroform. N—H stretching and bending vibrations are tabulated to liebetween 3200-3600 cm⁻¹ and 1450-1650 cm⁻¹ respectively. Carbonylstretching vibration of pure oleic acid is tabulated to be found at 1712cm⁻¹. The results indicated that oleate ligands originally attached tothe PbS QDs have been replaced by n-butylamine, indicated by the absenceof carbonyl stretching vibration, a significant shift of the N—Hstretching vibrations after exchange from 3294 and 3367 cm⁻¹ (Δ=73 cm⁻¹)for n-butylamine to 3610 and 3683 cm−1 (A=73 cm⁻¹), and the presence ofN—H bending vibrations for the n-butylamine exchanged sample.

FIG. 6D shows the FTIR spectra of inert-exchanged ligand-exchanged QDswith butylamine ligands before and after methanol wash, whichsubstantially removes the ligands from the QDs. Following methanol wash,features attributable to butylamine (1400, 1126, 989, 837, and 530 cm⁻¹)are much less pronounced. The inset also shows the N—H stretchingvibrations, which are again much less pronounced following methanolwash.

FIG. 6E shows spectra obtained by X-ray photoelectron spectroscopy (XPS)to confirm the material modifications that occur to the PbS QDsthroughout various processing steps. After background subtraction, thebinding energy was reference to the Cls hydrocarbon line at 285.0 eV.The curves were fitted by applying Gaussian-Lorentzian functions and theatomic ratios were obtained by integrating the areas under the signals.The nanocrystals immediatedly after exchange to butylamine ligandsdemonstrate a S2-peak at 160.7 eV corresponding to lead sulfide. No leadsulfate (PbSO₄) signal was detected. Nanocrystals that were precipitatedin air exhibit an SO₄ ⁻² at 167.5 eV characteristic of PbSO₄ formation.This oxide may be associated with the role of barrier to conductionamong nanocrystals. The ratio of PbS/PbSO₄ for this case was found to beabout 3.4:1. XPS of the inert-precipitated QDs after methanol soakingexhibits also formation of lead sulfate. The PbS/PbSO₄ ratio in thiscase was 18.6:1. Further annealing of this film in air at 120° C. for 1hour dramatically increased the amount of sulfate and the PbS/PbSO₄ratio was 2.44:1.

FIG. 6F shows the FTIR spectra of ligand-exchanged QDs precipitated ininert conditions (butylamine-called QDs) and precipitated in air-ambientconditions (oxidize-then-neck QDs). The inert-precipitated exchanged QDlayer after 2 hours of methanol wash (neck-then-oxidize QDs) are alsoshown. The broad feature around 1147 cm⁻¹ is attributed to PbSO₄ (leadsufate). The spectra show that ligand-exchanged QDs precipitated ininert conditions do not show this feature; methanol wash introduces someoxidation; ligand-exchanged QDs precipitated under an air ambient showevidence of strong oxidation. These results agree with the XPS dataabove.

Some performance characteristics of various representative deviceshaving different kinds of QD nanocrystal layers (e.g.,neck-then-oxidize, oxidize-then-neck, butylamine-capped, andneck-then-overoxidize) were measured. The general device structure alsoshown in the inset of FIG. 7A, and can be seen to generally be similarto that of FIG. 4B. The device included a transparent glass substrate;two gold electrodes having a length of about 3 μm, a width of about 5μm, and being spaced from each other by about 5 μm; and a QD nanocrystalof variable thickness between the electrodes.

Photoconduction was studied with the aid of optical excitation throughthe glass substrate, with excitation light being transmitted through thespace separating interdigitated electrodes, i.e., where the QD layer wasformed. The current-voltage characteristics for two different QDnanocrystal layer thickness are depicted in FIG. 7A, specifically theI-V characteristic for a “thin” 100 nm and a “thick” 500 nm QDnanocrystal layer devices. Photocurrents and dark currents respondlinearly to applied bias. The responsivity of the thick device reached166 A/W. The linear I-V characteristic indicates an ohmicelectrode-nanocrystal contact and suggests not a tunneling but a strong,direct conductive connection between QD nanocrystals. Photocurrent inthe thick device is significantly higher than the photocurrent of thethin device by virtue of greater absorbance in the thick device.

In order to determine optical power incident over the detector area andto calculate the responsivity R, a 2 mm radius beam from a 975 nm laserwas incident, first through a series of optical attenuators of knowntransmittance, and thence through the glass substrate, onto the devicefrom the back side. On the top surface, infrared-opaque interdigitatedgold electrodes were separated by 5 μm over a 3 mm path length. Theoptical power incident on the device was obtained by integrating theintensity profile of the laser over the unobstructed area of the device.Current-voltage characteristics were acquired using an Agilent 4155semiconductor parameter analyzer. The optical power impinging on eachdevice was about 80 pW.

The responsivity as a function of applied bias of devices made withdifferent kinds of QD nanocrystal layers is shown in FIG. 7B. Here, thenanocrystal layers were about 800 nm thick. The “neck-then-oxidize” QDdevice, corresponding to a device having a layer of fused QDs withdefect states on their outer surfaces, can clearly be seen to have asignificantly higher responsivity than the other devices. The“oxidize-then-neck” QD device, in which the ligands are removed from theQDs, and the QDs are fused, but in which the QDs are not maintained inan inert atmosphere between the steps of ligand removal and QD fusing,has defect states in the regions in which the QDs are joined thatreduces their responsivity, as compared with the “neck-then-oxidize”device, in which the QDs are maintained in an inert atmosphere betweenthe steps of ligand removal and QD fusing. All of the “necked” deviceshave a significantly higher responsivity than the device havingbutylamine capped QDs, in which the butylamine ligands block facileconduction of electrons between QDs.

In general, the responsivity of QD devices (particularly the “neck thenoxidize” QD devices) as measured in A/W is at least about 10 A/W, 100A/W, 1000 A/W, or even more than about 10000 A/W. The responsivity is afunction in part of the bias voltage applied, with a greaterresponsivity at higher bias. In some embodiments, the QD devices(particularly the “neck then oxidize” QD devices provide a substantiallylinear responsivity over 0-10 V with a bias applied across a distance of0.2 to 2 μm width or gap.

FIG. 7C shows the dark current density for the devices describedregarding FIG. 7B. As for the responsivity, “necked” devices have asignificantly higher dark current density than the device havingbutylamine capped QDs. Devices made using QDs exposed to oxygen beforenecking (“oxidize-then-neck”) show a superlinear I-V behaviorcharacteristic of field-assisted transport. In contrast, devices madeusing QDs fused before oxidation (“neck-then-oxidize”) exhibit linear(field-independent) behavior. Further oxidation of neck-then-oxidizedevices (“neck-then-overoxidize) leads to a decrease of conductivityowing to excessive oxide formation.

FIG. 7D shows the measured noise current as a function of the measureddark current for the devices described regarding FIG. 7B.“Neck-then-oxidize” devices exhibited the lowest noise current,approaching within 3 dB the shot noise limit. “Oxidize-then-neck”devices had the highest noise current, consistent with multiplicativenoise. “Neck-then-overoxidize” QD devices showed lower noise levels thanthe oxidize-then-neck QD devices although they contained larger amountsof oxide. This indicates the role of the oxidiation step in thefabrication process. The Johnson noise limit, the shot-noise limit, andthe fundamental background-limited thermodynamic (BLIP) noise current ofthe best-performing device (neck-then-oxidize) are also plotted forcomparison.

FIG. 7E shows a plot of the normalized detectivity D* as a function ofapplied bias. The normalized detectivity D* is measured in units ofJones (cmHz^(1/2)W⁻¹). D* is given as (AΔƒ)^(1/2)R/I_(n), where A is theeffective area of the detector in cm², Δƒ is the electrical bandwidth inHz, and R is the responsivity in AW⁻¹ measured under the same conditionsas the noise current i_(n) in A. The material figure of merit D* allowscomparison among devices of different powers and geometries. The devicefigure of merit, noise equivalent power (NEP)—the minimum impingingoptical power that a detector can distinguish from noise—is related toD* by NEP=(AΔƒ)^(1/2)/D*. As can be seen in FIG. 7E, the normalizeddetectivity D* is the highest for the “neck-then-oxidize” device, andthe lowest for the “oxidize-then-neck” device. In other words, allowingthe QDs to be exposed to oxygen after ligand removal and before neckingor fusing significantly affects the normalized detectivity of thefinished device. In the example devices shown, the normalizeddetectivity of the “neck-then-oxidize” device is more than an order ofmagnitude higher than that for the “oxidize-then-neck” device. Thehighest detectivity was found at a modulation frequency of 30 Hz, andreached 1.3×10¹³ jones at 975 nm excitation wavelength.

FIG. 7F shows the results of further measurements of the“neck-then-oxidize” device described above regarding FIG. 7B. Thespectra of responsivity and the normalized detectivity D* is shown forthe “neck-then-oxidize” device at an applied bias of 40 V and anelectrical frequency of 10 Hz. D* was measured to be 1.8×10¹³ jones atthe excitonic peak wavelength. FIG. 7G shows the electrical frequencyresponse of the same device under 40 V bias. The 3-dB bandwidth of thedetector is about ˜18 Hz, consistent with the longest excited-statecarrier lifetime in the device. High sensitivity (D*>10¹³ jones) isretained at imaging rates of about 30 frames per second.

FIG. 7G shows the photocurrent temporal response following excitation ofthe QD layer of the “neck-then-oxidize” device of FIG. 7B, where theexcitation was a 7 ns pulse centered at 1064 nm, at a frequency of 15Hz. This allows investigation of the transit time and distribution ofcarrier lifetimes in the device. The response of the detector to theoptical pulse was found to persist over tens of milliseconds,attributable to the longest-lived population of trap states introducedby oxidation. The response exhibits multiple lifetime components thatextend from microseconds (though shorter components may exist they arenot observable in this measurement) to several milliseconds. Decaycomponents were found at about 20 μs, about 200 μs, about 2 ms, about 7ms, and about 70 ms. A transit time of about 500 ns was obtained for abias of about 100 V, revealing that transit time depends linearly onbias with a slope corresponding to a mobility of about 0.1 cm²V⁻¹s⁻¹.The ratio of the longest component of carrier lifetime to the transittime was thus on the order of about 10,000. The observed responsivity ofabout 2700 A/W in this example could thus be explained byphotoconductive gain, given the films absorbance of 0.3 at an opticalwavelength of 975 nm. This high responsivity was observed underlow-level optical power conditions relevant to ultrasensitive detection.In general, in some embodiments, as illumination intensity wasincreased, the longest-lived trap states became filled, and shorterlived, so lower-gain trap states began to account for a significantcomponent of carrier lifetime. The devices of these embodiments werethus highly sensitive under low-light conditions, and exhibit intrinsicdynamic-range-enhancing gain compression under increasing illuminationintensity.

For determining the photocurrent spectral response, a bias of 50 V wasapplied to the sample connected in series with a 100 Ohm load resistor.Illumination was provided by a white light source dispersed by a Triax320 monochromator and mechanically chopped at a frequency of ˜100 Hz.Filters were used to prevent overtones of the monochromator's gratingfrom illuminating the sample. The voltage across the load resistor wasmeasured using a Stanford Research Systems SR830 lock-in amplifier. Theintensity through the monochromator at each wavelength was measuredseparately using a calibrated Ge photodetector. The photocurrent at eachwavelength was subsequently scaled accordingly. After the photocurrentspectral shape was determined in this way, the absolute responsivity at975 nm was used to obtain the absolute spectral response 800 nm-1600 nm,which is shown in FIG. 8.

For measurement of noise current and calculation of NEP and D*, thephotoconductive device was placed inside an electrically-shielded andoptically-sealed probe station and connected in series with a StanfordResearch SR830 lock-in amplifier. Alkaline batteries were used to biasthe device for the measurement of the noise current in order to minimizenoise components from the source. The lock-in amplifier measured thecurrent in the photodetector and reported noise current in A/Hz^(1/2).Special care was taken in choosing an appropriate passband in order toacquire stable and meaningful measurements of noise current at variousfrequencies. This measurement revealed a significant increase in thenoise current below 5 Hz which is attributed to 1/f noise, while whitenoise patterns are observed above 50 Hz. The noise current divided bythe responsivity under the same measurement conditions of applied biasand frequency modulation yielded the noise equivalent power (NEP). Thenormalized detectivity, D*, was obtained as a function of wavelength,applied bias, and frequency by dividing the square root of the opticallyactive area of the device by the NEP.

To validate the NEP values obtained using this technique, the identicalprocedure was preformed using a commercial Si detector with known NEP.The system described above reported NEP values of the same order ofmagnitude, but typically somewhat larger than, the specified NEPs. TheNEP and D* determination procedure used herein thus provides aconservative estimate of these figures of merit.

FIG. 8 shows spectral dependence of responsivity and normalizeddetectivity D* for biases of 30, 50, and 100 V, under 5 Hz opticalmodulation and 0.25 nW incident optical power. The responsivity shows alocal maximum near 1200 nm corresponding with the exciton absorptionpeak of the nanocrystal solid-state films shown in the inset of FIG. 8.The responsivity increases with voltage (but not as rapidly as does thenoise current, resulting in higher D* at lower biases) and reaches 180A/W at 800 nm. For 30 and 50 V of applied bias, D* is 2×10¹¹ Jones andis more than double the detectivity of commercial polycrystalline PbSdetectors which have benefited from 50 years of science and technologydevelopment. Though the responsivity is higher at 100 V, thebias-dependence of the measured noise current results in D* beingmaximized at the lower bias of 30 V.

FIG. 9 shows the frequency dependence of responsivity and normalizeddetectivity for three values of applied bias at 975 nm and 0.25 nW ofincident optical power. The 3 dB bandwidth of the device responsivitywas 15 Hz for 100 V and 50 V and 12 Hz for 30 V. The measurements weretaken with optical excitation from a 975 nm laser and incident opticalpower 0.2 nW. The noise current was also measured for the threedifferent biases throughout the frequency range. The noise current wassignificantly higher at frequencies below 20 Hz, whilefrequency-independent white noise was observed at higher frequencies.Noise equivalent exposure, or NEE, is another way of expressing thelowest amount of light detectable by a detector. NEE is defined to bethe number of joules of optical energy that can create a signal at thedetector that is equivalent in magnitude to the noise on the detector,and is calculated to be the RMS noise on the detector, divided by theresponsivity of the detector. FIG. 10 shows the NEE of a QD devicehaving a layer of fused QDs with defect states (e.g., oxidation) ontheir outer surface, as compared with the NEE of a conventional Si CCDdetector as well as a conventional Si CMOS detector. The QD device hasan NEE of less than 10⁻¹¹ J/cm² at wavelengths of 400 to 800 nm, andfurther less than 10⁻¹⁰ J/cm² at wavelengths of 400 to 1400 nm. The NEEsof the conventional Si devices are significantly higher than that of theQD device, in some cases more than an order of magnitude higher.

The figures of merit obtained from the quantum dot detectors presentedherein result from a combination of processing procedures. First, theshortening of the distance between QDs via exchange to a much shorterorganic ligand provided enhanced inter-QD conduction. Post-depositiontreatment using a nonsolvent and exposure to elevated temperatures in anoxygen-rich atmosphere enabled further ligand removal, QD fusing, andthe formation of a native oxide on the QD surface. This oxide haspreviously been shown in polycrystalline PbS devices to be useful inachieving high D* in photoconductors. However, chemical bath-grownpolycrystalline devices with 200 nm domain sizes do not allow refinedcontrol over interfaces. In contrast, using pre-fabricated, highlymonodisperse, individually single-crystal QDs with highly-controlledligand-passivated surfaces to fabricate optical devices allowsexceptional control over interface effects compared withpolycrystalline-based devices. The quantum dot optical devices describedherein are superior across many figures of merit to conventionalgrown-crystal semiconductor optical devices. At the same time thefabrication of the devices is strikingly simple, while maintainingoptical customizability based on the quantum size effects of quantumdots.

Alternative Embodiments

Although the QDs are solution-deposited in the described embodiments,the QDs may deposited in other ways. As mentioned above, one motivationfor using solution-deposition is its ready compatibility with existingCMOS processes. However, satisfactory devices can be made byvacuum-depositing or otherwise depositing the QDs.

Other embodiments are within the following claims.

INCORPORATED REFERENCES

The following references, in some cases referred to above as the“incorporated references,” are incorporated herein by reference in theirentireties.

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1. A method of forming a nanocrystalline film, the method comprising:fabricating a plurality of nanocrystals, the nanocrystals having a coreand an outer surface, a plurality of first ligands having a first lengthbeing attached to the outer surface; exchanging the plurality of firstligands attached to the outer surface of the nanocrystals for aplurality of second ligands having a second length and having adifferent chemical composition than the plurality of first ligands;forming a film of ligand-exchanged nanocrystals, at least a portion ofthe ligand-exchanged nanocrystals being adjacent at least one otherligand-exchanged nanocrystal; and removing the second ligands attachedto the outer surfaces of the nanocrystals of the film ofligand-exchanged nanocrystals so as to bring the outer surfaces ofadjacent nanocrystals into closer proximity; and forming an electricalnetwork of nanocrystals.